Effective mixing in supersonic and hypersonic flow conditions is critical for developing next-generation high-speed air-breathing transport systems. Efficient fuel mixing with fast-moving air leads to a better economy and fewer pollutants. Ultimately the mixing is a molecular diffusion problem. However, the macroscopic phenomena, such as entrainment
and vorticity dynamics resulting from the shear layer instabilities of the mixing fluids, play a significant role in the overall efficiency of the process. This paper studies a novel, coaxial injector system integrated with a high-frequency microjet actuator operating at 15.5 kHz for improving mixing in extreme flow conditions. This co-flow system consists of a highfrequency supersonic actuation air jet at the inner core that provides large mean and fluctuating velocity profiles in the shear layers of a fluid stream injected surrounding the core through an annular nozzle. The high-frequency streamwise vortices and shockwaves tailored to the mean flow significantly enhanced supersonic flow mixing between the fluids compared to a steady co-axial configuration operating at the same input pressure. This paper reports the flow mixing characteristics of the injection system captured using planar laser-induced fluorescence (PLIF).
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High-Frequency Pulsed Co-axial Injectors for High-Speed Flow Mixing and Control
Efficient and controlled mixing of fuel with fast-moving air is a challenging physical problem relevant to hypersonic systems. Although mixing happens at the molecular level through diffusion, the macroscopic phenomena such as entrainment and vorticity dynamics resulting from the shear layer instabilities of the mixing fluids play a significant role in the overall efficiency of the process. With a focus on improving mixing at extreme flow conditions, this paper presents a fundamental study of a novel, high-speed, pulsed co-flow system integrated with ultra-high frequency actuators that operates at 11-20 kHz. This injection system consists of a supersonic actuation air jet at the inner core that provides large mean and fluctuating velocity profiles in the shear layers of a fluid stream injected surrounding the core through an annular nozzle, with pulsing occurring at a designated frequency. The high-frequency streamwise vortices and shockwaves tailored to the mean flow significantly enhanced supersonic flow mixing between the fluids compared to a steady co-axial configuration operating at the same input pressure. Experiments also indicate a strong connection between the frequency and unsteady amplitude of the actuation jet to the supersonic flow mixing phenomena. This paper reports the design details of the injector assembly and flow mixing characteristics captured using phase-locked microschlieren and planar laser-induced fluorescence (PLIF) techniques.
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- Award ID(s):
- 1900177
- NSF-PAR ID:
- 10337919
- Date Published:
- Journal Name:
- AIAA Aviation Meeting Chicago 2022
- Volume:
- 3926
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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This paper presents the velocity and vorticity fields of a supersonic, pulsed co-flow injection system, measured using particle image velocimetry (PIV). The active injection system consists of an actuation air jet from a nozzle (1 mm ID, 1.5 mm OD) that provides large mean and fluctuating velocity profiles to the shear layers of a fluid stream injected through a 460 μm diameter annular space surrounding the nozzle. The injector assembly is designed to operate in three modes in the present study. In the first case, annular fluid was injected at 70 m/s without actuation and treated as baseline flow. In the second case, a steady under-expanded supersonic jet with an exit velocity of 350 m/s interacts with the injected annular fluid. In the last case, the actuation jet pulsing at 15.5 kHz introduces highfrequency streamwise vortices and shockwaves to the annular stream. PIV data indicate that steady and pulsed actuation significantly modifies the velocity and vorticity fields of annular flow. While steady actuation caused intensive shear and high vorticity streaks in the flow, the high-frequency vortex generated in pulsed actuation resulted in highly fluctuating velocity and vorticity fields that favor enhanced mixing between the annular stream and supersonic actuation jet. Quantitative data from this study confirm the potential of this injector system for flow mixing and control under extreme conditions.more » « less
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With an increased national emphasis on the development of high-speed systems, especially those utilizing air-breathing propulsion, there is a strong demand for efficient means of enhancing the mixing of fuel injection and an air stream that may be moving at supersonic speeds. In recent years, small scale fluidic-based actuators have shown promise in their ability promote efficient mixing in these scenarios due to their small size and having no moving parts. High-frequency pulsed co-axial actuators developed at Tuskegee University are one such device that only rely on a compressed gas source and particular geometric designs to enable rapid pulsed injection. These actuators are able to efficiently atomize injected fluids, such as fuels or oxidizers, and they can be designed to pulse at frequencies in excess of 15 kHz. This paper presents the results of an experimental investigation of one such device using advanced optical and laser diagnostics to measure both the spectral content and the unsteady velocity fields of the pulsed jet output. The flowfield is visualized with high-speed schlieren imaging, and the near-field acoustic emission is measured by a microphone. Focused laser differential interferometry (FLDI) and planar particle image velocimetry (PIV) are conducted within the actuator’s pulsed jet output. The spectral content measured from FLDI closely matches the acoustic spectra. While the phase-locked velocity fields demonstrate the pulsing behavior of the actuator’s core flow jet, the calculated velocities appear to be lower than what was anticipated based on inspection of the high-speed schlieren images. Velocity fields are presented for the actuator operating in both steady and pulsed modes of operation, and the results indicate that the pulsed actuation jet has the potential to significantly enhance mixing of the annular stream when it is introduced.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.2514/6.2022-3926
